For narrow beams at higher frequencies transmission and reception schemes will be needed in order to compensate for high propagation losses that will occur. A suitable Transmission Reception Point (TRP) transmission (TX) beam for each user equipment (UE) is expected to be discovered and monitored by a communications system using measurements on downlink reference signals used for beam management. The CSI-RS (channel state information-reference signal) is the reference signal that has been agreed on in 3rd Generation Partnership Project (3GPP) as beam reference signal for New Radio (NR). The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodically (event triggered) and it can either be shared between multiple UEs or be UE-specific. In order to find a suitable TRP beam the TRP transmits CSI-RSs in different TRP TX beams on which the UE performs Reference Signal Received Power (RSRP) measurements. The UE reports back the N best TRP TX beams, where N can be configured by the communications system.
There are basically three different implementations of beamforming at the TRP: analog beamforming, digital beamforming and hybrid beamforming. Each implementation has its advantages and disadvantages. Digital beamforming is the most flexible solution but also the most expensive due to the large number of radios and baseband chains that are required. Analog beamforming is the least flexible but also less expensive to manufacture owing to a reduced number of radio and baseband chains compared to the digital solution. Hybrid beamforming is a compromise between the analog and digital beamforming. The concept of antenna panels is one type of beamforming antenna architecture at the TRP that is to be studied in 3GPP for the NR access technology. An antenna panel is an antenna array of single- or dual-polarized elements with typically one transmit/receive unit (TXRU) per polarization. An analog distribution network with phase shifters is used to steer the beam of each panel.
In today's communications systems, the traffic profile is such that a majority of data sessions are very short. However, there is still a significant part of the traffic that comprises longer data sessions, such as, for instance, streaming of music, videos and voice calls etc. and the capacity demand in the communications system may then be high. One drawback with the analog beamforming implementation is that the TRP can only transmit or receive in one beam at a time, if assuming one panel and the same beam for both polarizations, which is typically the case in order to counteract dropped signal strength due to polarization mismatching. Further, the TRP antennas (e.g. a panel) for this implementation can only transmit with one beam at each time instant. If the beams are narrow, this reduces the possibility to serve multiple UEs simultaneously since it is unlikely that two UEs that have data in their buffers will be within the same beam. There may thus be a lack of capacity in the communications system, and an increased latency, both of which decrease user satisfaction.